A conventional vapor compression cycle device for refrigeration, air-conditioning or heat pump purposes is shown in principle in FIG. 1. The device consists of a compressor 1, a condensing heat exchanger 2, a throttling valve 3 and an evaporating heat exchanger 4. These components are connected in a closed flow circuit, in which a refrigerant is circulated. The operating principle of a vapor compression cycle device is as follows: The pressure and temperature of the refrigerant vapor are increased by the compressor 1, before it enters the condenser 2 where it is cooled and condensed, giving off heat to a secondary coolant. The high-pressure liquid is then throttled to the evaporator pressure and temperature by means of the expansion valve 3. In the evaporator 4, the refrigerant boils and absorbs heat from its surroundings. The vapor at the evaporator outlet is drawn into the compressor, completing the cycle.
Conventional vapor compression cycle devices use refrigerants (as for instance R-12, CF operating entirely at subcritical pressures. A number of different substances or mixtures of substances may be used as a refrigerant. The choice of refrigerant is, among others, influenced by the condensation temperature, as the critical temperature of the fluid sets the upper limit for the condensation to occur. In order to maintain a reasonable efficiency, it is normally desirable to use a refrigerant with a critical temperature at least 20-30K above the condensation temperature. Near-critical temperatures are normally avoided in design and operation of conventional systems.
The present technology is treated in full detail in the literature, e.g. the Handbooks of American Society of Heating, Refrigerating and Air Conditioning Engineers Inc., Fundamentals 1989 and Refrigeration 1986.
The ozone-depleting effect of presently employed common refrigerants (halocarbons) has resulted in strong international action to reduce or prohibit the use of these fluids. Consequently there is an urgent need for finding alternatives to the present technology.
Control of the conventional vapor compression cycle device is achieved mainly by regulating the mass flow of refrigerant passing through the evaporator. This is done, e.g., by suction line throttling or bypassing the compressor. These methods involve more complicated flow circuit and components, a need for additional equipment and accessories, reduced part-load efficiency and other complications.
A common type of liquid regulation device is a thermostatic expansion valve which is controlled by the superheat at the evaporator outlet. Proper valve operation under varying operating conditions is achieved by using a considerable part of the evaporator to superheat the refrigerant, resulting in a low heat transfer coefficient.
Furthermore, heat rejection in the condenser of the conventional vapor compression cycle device takes place mainly at constant temperature. Therefore, thermodynamic losses occur due to large temperature differences when giving off heat to a secondary coolant with a large temperature increase, as in heat pump applications or when the available secondary coolant flow is small.
The operation of a vapor compression cycle device under transcritical conditions has been formerly practiced to some extent. Up to the time when the halocarbons took over, 40-50 years ago, CO.sub.2 was commonly used as a refrigerant, notably in ship refrigeration systems for provisions and cargo. The systems were designed to operate normally at subcritical pressures, with evaporation and condensation. Occasionally, typically when a ship was passing tropical areas, the cooling sea water temperature could be too high to effect normal condensation, and the plant would operate with supercritical conditions on the high side. (Critical temperature for CO.sub.2 31.degree. C.). In this situation it was practiced to increase the refrigerant charge on the high side to a point where the pressure at the compressor discharge was raised to 90-100 bar, in order to maintain the cooling capacity at a reasonable level. CO.sub.2 refrigeration technology is described in older literature, e.g. P. Ostertag "Kalteprozesse", Springer 1933 or H. J. MacIntire "Refrigeration Engineering", Wiley 1937.
The usual practice in older CO.sub.2 -systems was to add the necessary extra charge from separate storage cylinders. A receiver installed after the condenser in the normal way will not be able to provide the functions intended by the present invention.
Another possibility is known from German Patent No. 278,095 (1912). This method involves two-stage compression with intercooling in the supercritical region. Compared to the standard system, this involves installation of an additional compressor or pump, and a heat exchanger.
The textbook "Principles of Refrigeration" of W. B. Gosney (Cambridge Univ. Press 1982) points at some of the peculiarities of near-critical pressure operation. It is suggested that increasing the refrigerant charge in the high-pressure side could be accomplished by temporarily shutting the expansion valve, so as to transfer some charge from the evaporator. But it is emphasized that this would leave the evaporator short of liquid, causing reduced capacity at the time when it is most wanted.